Conventional methods for additional decarburization refining of a molten steel which has been once subjected to decarburization refining in an electric furnace or a converter to provide a molten steel having a carbon concentration of not more than 0.01% by weight include: (1) a VOD (vacuum oxygen decarburization) method, typified by the one disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-43924, wherein an oxygen gas is blown onto the surface of a molten steel in a ladle while holding the molten steel surface in vacuo; and (2) a straight barrel type snorkel method wherein an oxygen gas is blown onto the surface of a molten steel within a snorkel submerged in molten steel to carry out vacuum refining.
In the method (1), VOD, a satisfactory space cannot be ensured above the molten steel surface. This causes a splash of molten steel, scattered during oxygen blowing decarburization refining, to be deposited onto a top-blown lance and a cover of a vacuum vessel, adversely affecting the operation.
The method (2), straight barrel type snorkel method, unlike the method (1), has no significant limitation on equipment, and an example of this method is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-37912. The method disclosed in this publication is shown in FIG. 35. Specifically, in this method for vacuum refining of molten steel, a molten steel 71 contained in a ladle 70 is sucked through a snorkel 72 into a vacuum tank 73. An inert gas is blown into the molten steel within the snorkel 72 through under the plane of projection of the snorkel 72 within the ladle 70, and, at the same time, an oxidizing gas is blown through a top lance 74 onto the surface of the molten steel within the vacuum tank 73. In this case, the inner diameter of the snorkel 72 is determined so that the ratio of the inner diameter (D.sub.1) of the snorkel 72 to the inner diameter (D.sub.0) of the ladle 70, that is, D.sub.1 /D.sub.0, is 0.4 to 0.8. In addition, the depth of blowing of the inert gas is determined so that the ratio of the depth (H.sub.1) of blowing of the inert gas as measured from the surface of the molten steel to the depth (H.sub.0) of the molten steel within the ladle 70, that is, H.sub.1 /H.sub.0, is 0.5 to 1.0. The above method for vacuum refining of molten steel aims to efficiently carry out decarburization without the deposition of the metal, slag and the like within the tank.
Japanese Unexamined Patent Publication (Kokai) No. 2-133510 proposes a vacuum treatment apparatus comprising: a ladle for placing therein a molten metal; a vacuum tank having a snorkel, submerged in the molten metal, provided at the lower end of the vacuum tank; an evacuation pipe connected to a vacuum source for evacuating the interior of the vacuum tank; and a shield disposed in the interior of the vacuum tank, wherein the shield is kept at a height of 2 to 5 m above the molten steel surface within the snorkel.
The method proposed in Japanese Unexamined Patent Publication (Kokai) No. 61-37912, however, had the following problems (i) to (iv).
(i) Conditions for decarburization refining, such as the flow rate of the oxygen gas blown onto the molten steel, the flow rate of the argon gas for agitation, and the degree of vacuum within the vacuum tank 73, are not properly specified. This causes excessive fluctuation of the molten steel surface and splashing, leading to operation troubles attributable to deposition of the metal.
(ii) In the oxygen blowing decarburization refining of chromium-containing molten steel, such as stainless steel, the chromium component contained in the molten steel is oxidized with the blown oxygen. A part of the chromium oxide produced by the oxidation is reduced with carbon contained in the molten steel in the course of descending through the molten steel. Most part of the chromium oxide, however, undergoes the convection due to the inert gas blown from below the molten steel and floats, without being reduced, on the surface of the molten steel between the snorkel and the inner wall of the ladle to form slag 75 which is then discharged from the molten steel, increasing the loss of the chromium component.
(iii) The presence of the slag 75 containing chromium oxide causes the surface of the molten steel present between the snorkel 72 and the inner wall of the ladle to come into contact with air and to be cooled. This increases the viscosity of the molten steel surface. In addition, the slag 75, the metal or the like is deposited around the above inner wall of the ladle, making it difficult to conduct sampling of the molten steel in the course of and at the end of the refining, or making it difficult to move the snorkel 72 from the position of the ladle 70 at the end of the refining, which is an obstacle to refining.
(iv) The oxygen efficiency in the decarburization, defined as the ratio of the amount of the oxygen gas contributed to the decarburization of the molten steel to the total amount of the oxygen gas blown onto the molten steel, is influenced by refining conditions, such as the degree of vacuum in the vacuum tank 73, the state of agitation of the molten steel, and the flow rate of the oxygen gas blown. These refining conditions are not proper, making it difficult to maintain the oxygen efficiency in decarburization at a high level.
The method described in Japanese Unexamined Patent Publication (Kokai) No. 2-133510, wherein a shield is provided within a vacuum tank (a snorkel) to prevent splash of the molten steel created by oxygen blowing, thereby preventing deposition and accumulation of a metal caused by solidification of the splash deposited onto an oxygen lance, a vacuum tank, an evacuation pipe, had the following problems.
(i) When an exhaust gas is passes between shields within the vacuum tank, the molten steel splash in the exhaust gas or dust produced by solidification of the splash is deposited and accumulated onto the shields, increasing the flow resistance of the exhaust gas, which in turn increases the pressure loss within the vacuum tank.
(ii) Since the spacing, between the shields, serving as a passage for the exhaust gas becomes narrow, a high-power evacuation apparatus is necessary to provide a high degree of vacuum.
(iii) When a metal or the like scattered by splashing or spitting is once deposited and accumulated onto the passage for the exhaust gas between the shields, removal of the deposited and accumulated metal cannot be achieved without difficulty due to the complicated structure and requires a lot of time and labor.
In the method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61-37912, when the oxygen blowing refining is carried out at a high speed in order to increase the productivity of vacuum refining, the splashing is remarkably increased, posing the following problems which will be described with reference to FIG. 35.
(i) Although the creation of the splash of the molten steel 71 per se can be inhibited, dust is still contained in the exhaust gas. Therefore, the dust is gradually deposited within the evacuation duct 76 particularly around its duct inlet section to form a deposit 77, clogging the passage or increasing the air-flow resistance, which lowers the attainable level of the degree of vacuum within the vacuum tank 73.
(ii) Dust is introduced into a gas cooler 78 and damages the gas cooler. This results in suspension of equipment and increased maintenance cost. Further, a dust deposit is formed within the gas cooler 78, which causes a markedly lowered cooling efficiency.
(iii) Once a dust deposit 77 is formed within an evacuation duct 76, the dust is strongly united and must be manually removed. This increases the dust removal burden.
The method described in Japanese Unexamined Patent Publication (Kokai) No. 61-37912 is disadvantageous in that, for example, chromium oxide (Cr.sub.3 O.sub.3) formed during oxygen blowing decarburization flows out from the snorkel into the outside of the vacuum tank and, since Cr.sub.2 O.sub.3 has a high melting point, slag on the ladle is solidified, making it difficult to sample the molten steel, that is, posing a problem in the operation. An additional problem involved in this method is that Cr.sub.2 O.sub.3, which has once flowed out into the outside of the tank, does not contribute to a later decarburization reaction, inevitably resulting in lowered oxygen efficiency in decarburization.
RH--OB is widely known as a method for oxygen blowing decarburization refining in vacuo. When this method is used, for example, in the finishing of stainless steel, aluminum is added to the molten steel before the oxygen blowing decarburization and combustion is carried out using top-blown oxygen to raise the temperature of the molten steel (aluminum temperature elevation or temperature elevation by aluminum). In this case, when aluminum temperature elevation is carried out under a high degree of vacuum, the depth of a cavity, of the molten steel, formed by a blown oxygen jet (cavity depth) becomes large, leading to a fear of bricks at the bottom of the tank being damaged by the blown oxygen jet, which makes it difficult to conduct temperature elevation by aluminum under a high degree of vacuum.
Further, the straight barrel snorkel type vacuum refining method is disadvantageous in that, as can be seen in the process for producing an ultra low carbon high chromium steel disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-43924, there is a limitation on the decarburization in a degassing period due to the difficulty of maintaining the agitating force and, as can be seen in the vacuum refining method disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-305917, an attempt to improve the reduction rate in the degassing period results in remarkable wear of refractories.
Furthermore, after the oxygen blowing decarburization, introduction of aluminum as a reducing agent into the molten steel within the vacuum tank in order to recover a metal by reduction of a metal oxide, for example, chromium oxide, causes a rise in temperature of the molten steel by heat generated by thermit reaction, or scattering (bumping) of the molten steel or slag by a reduction reaction involving instantaneous evolution of CO gas, resulting in melt loss of refractories within the tank and deposition of the metal or slag, which is an obstacle to the operation.